RoCE vs TCP for Low-Latency Apps

The effectiveness of our communication as a species is one of our defining characteristics. Earlier this week while waiting in a customer’s lobby in Chicago I noticed four framed posters displaying all the hand signals used in the trading pits of four major markets. Having been focused on electronic trading for the past decade this “ancient” form of communications became an instant curiosity worthy of inspection. On reflection, I was amazed to think that trillions of dollars in transactions over decades had been conducted purely by people motioning with their hands.

About a decade ago in the High-Performance Computing (HPC) market, a precursor market for High-Frequency Trading (HFT), there was a dust-up regarding the effectiveness of Remote Direct Memory Access (RDMA). One of Myricom’s senior researchers wrote an article for HPCWire titled “A Critique of RDMA” that set off a chain reaction of critical response articles:

At the time Myricom was struggling to establish relevance for its new Myrinet-10G protocol against a competing technology, Infiniband, which was rapidly gaining traction. Now to be fair, at the time I was in sales at Myricom. The crux of the article was that the one-sided RDMA communications model, which rose from the ashes of the Virtual Interface Architecture (VIA), was still more of a problem than a solution when compared to the existing two-sided Send/Recv model used by four other competing HPC protocols (QsNet, SeaStar, Infinipath & Myrinet Express).

Now RDMA has had a decade to improve as it spread from Infiniband to Ethernet under the name RDMA over Converged Ethernet (RoCE), but it still has performance issues. The origin of RDMA is cast in a closed lossless layer-2 Infiniband network with deterministic latency. Let’s take a moment and adopt a NASCAR analogy. Think of RDMA as the vehicle and Infiniband as the track. One can take a Sprint Cup Series vehicle tuned for the Charlotte Motor Speedway, and take it for a spin on the local roads, but is that really practical (it certainly isn’t legal)? Yes, its origin is in the stock car, but how well will it do in stop and go traffic, particularly on uphill grades? How about parallel parking, oh wait there’s no reverse. Tight turns at low speeds, signaling, weather, etc. Sprint Cup Series vehicles are designed for 200MPH on a closed extremely well defined and maintained course. Ethernet, by contrast, is the road driven by everyone else, it’s unpredictable with thousands of obstacles, and is ever changing.

Those familiar with Ethernet know that lossless and deterministic latency are not two characteristics often normally associated with this network fabric. Some of us have been around the block and lived through Carrier Sense Multiple Access with Collision Detection (CSMA/DA) where packets often collided and random delays before retransmission attempts were common. TCP/IP was developed during these early days and it was designed with this packet loss as a key criterion. In the past three decades Ethernet has evolved considerably from its roots as a shared coax cable utilizing vampire taps to where we are today with dedicated twisted pair cabling and fiber optics, but on rare occasion, packets are still dropped, and performance isn’t always deterministic. Today most packet drops are as a result of network congestion. As discussed TCP/IP is equipped to handle this, unfortunately, RoCE is not.

For RoCE to perform properly it requires a lossless layer-2 network. Essentially a NASCAR track overlaid onto our public roads. To accomplish this over a routed Ethernet network a new protocol was developed: Data Center Bridging Capabilities Exchange (shortened to DCB or DCBX). DCB is used at every hop of the network to negotiate and create a lossless layer-2 fabric on top of Ethernet. It achieves this by more tightly managing queue overflows and by adjusting network flow priorities as if they were traversing separate physical media. In essence RoCE traffic is prioritized into essentially its own carpool lane ahead of other traffic in hopes of avoiding drops as a result of congestion. While this all sounds great, in talking with several large Web2.0 customers who’ve invested years in RoCE we learned that the vast number will never deploy it in production. There are far too many challenges to get and keep it working, and in high traffic volumes, it suffers. Unlike Infiniband HPC clusters which are stood up as self-contained networks (closed course race tracks) to address specific computational problems, Ethernets are in a constant state of flux with servers and switches being added and removed (our public road system) as the needs of the business change. To be clear TCP/IP is resilient to packet loss, while RoCE is not.

On the latency performance side of things, in the past decade, we’ve achieved roughly one microsecond for a 1/2 round trip (a send + receive) with both TCP and UDP, when using Solarflare’s OpenOnload. This is in line with RoCE latency which is also in the domain of one microsecond. Keep in mind that normal TCP or UDP transactions over 10GbE typically run in the range of 5 to 15 microseconds, so 1 microsecond is a huge improvement. By now you’re likely saying “So what?” For most applications like file sharing, databases, etc… the difference between one microsecond and even fifteen microseconds is lost in the 10,000+ microseconds a whole transaction might take. It turns out though that there are new breeds of network latency-sensitive applications that depend on technologies like Non-Volatile Memory Express (NVMe), Neural Networks, and high volume compound web transactions that can see significant improvements when latency is reduced. When low latency TCP is applied to these problems the performance gains are both measurable and significant.

So the next time someone suggests RoCE ask if they’ve considered a little known competing protocol called TCP/IP. While RoCE is the shiny new object, TCP/IP has had several decades of innovation behind it which explains why it’s the underlying “language of the Internet”. Consider asking those promoting RoCE what their porting budget is, and if they’ve factored in the cost of the new network switches that will be required to support DCB? It’s very likely that the application they want to deploy already supports TCP/IP, and if latency and throughput are key factors then consider contacting Solarflare about OpenOnload. OpenOnload accelerates existing sockets based applications without having to modify them.

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One thought on “RoCE vs TCP for Low-Latency Apps”

I have been out of the industry for nearly 10 years and it took me less than 15 minutes to figure out how RDMA/RoCE should be deployed. What you are complaining about is not what it was intended for (despite what some commissioned sales reps might say). RoCE is clearly a fast connection to a SAN and maybe as a direct connection between a few servers. Anyone trying to do anything else with it should be fired (unless they are inexperienced) because they clearly dont know how to read or understand specifications.

From reading your article, it almost sounds like you are comparing a server room network built on RoCE vs one that uses IP. Thats not what its optimized for and that is made clear with a just a quick glance at the wiki. IP is for large scale (as it always has been). RoCE is for isolated high performance connections. It is meant to replace old FC that we used 10 years ago. iWarp/RDMA is an intermediary between IP and RoCE but not as good as the other two at any specific job (scale vs performance) which leaves it out in the cold. This is probably why Chelsio marketing seems to sound so desperate and hostile towards Mellonox.